Making Buildings Airtight & Recovering Escaping Heat

In the past, the heating and ventilation of buildings have tended to be considered as separate concerns that come into conflict. As fossil fuel prices rise, the need for energy efficiency in achieving both is increasingly leading Irish people to an approach which combines both ventilation and heating, through making buildings airtight and recovering heat from outbound air.

Air Tightness

Homeowners and businesses are constantly looking at options to reduce the running costs of their buildings. With fuel prices rising and uncertainty in the market it makes sense to minimise the amount of heating required for any property. The air tightness performance of a building can play a huge part in reducing heating requirements.

The term itself – air tightness – is a somewhat confusing one – perhaps air control is more accurate. When we talk of air tightness, what we’re essentially speaking about is the elimination of draughts. In other words when we want fresh air, we open a window or slide the cover of a vent across, and we have the fresh air that we want. Draughty buildings provide fresh air whether we want it or not. In winter when ambient external temperatures may be only three or four degrees and we like to relax indoors in temperatures around twenty degrees, we end up footing the bill for warming-up any incoming air. The less cold air that we have to heat the better – so air tightness, or air control saves money. Airtight buildings offer another economic benefit – they don’t let much warm air escape either.

But ventilating a building can be a more sophisticated procedure than opening or closing a window. Heat recovery ventilation is a process where you not only ventilate but save energy, with heat removed from the air leaving a building and added to the incoming cold air without cross contamination. You still have to add a bit of heat to make up for the loss in the exchanger – although some systems claim in excess of 90% efficiency – and also to compensate for heat loss every time somebody opens an external door. There’s also the heat lost through the walls and roof – even with insulation.

Any small openings which exist in the building, such as around window frames, pipes, and so on, will allow air through, in the form of draughts. Currently our Building Regulations stipulate that we have purpose built openings in our walls to allow cold air to go through our homes. These openings are deemed unsightly, uncomfortable, noisy, impractical and highly inefficient. The latest thinking on this subject is to provide a system of building air tightness combined with a heat recovery ventilation system.

Timber frame buildings use a membrane underneath the plaster slabs known as a vapour check, to prevent moisture from entering the building structure. This can easily be upgraded to an air tightness membrane standard by the use of sealing tapes, mastics and so on, which are readily available. The airtightness of concrete buildings can be improved by sealing around doors, windows, and floor-to-wall and wall-to-ceiling joints. The air tightness performance can then be tested using what is known as a blower door test.

In a blower door test all of the windows and external doors are closed and any vents closed off. An apparatus fitted to an external door opening will effectively pump up the house to a certain small pressure and then measure the time taken to normalise. This test will reveal any air leakage points which can then be sealed.

To stay healthy in this sealed environment, we need to take in oxygen, get rid of carbon dioxide, and also expel much of the water vapour which we constantly produce. To do this we need a balanced ventilation system. This is a system which takes in a similar amount of air to that taken out. The problem with a simple fan system is that the air being taken out from bathrooms and kitchen is warm and the air drawn in to bedrooms and living rooms is cold. To overcome this, we bring the two airstreams through a heat exchanger which can recover heat from exiting air and transfer it to incoming air. This, when connected to ductwork throughout the house, is termed a heat recovery ventilation system (HRV).

When we seal our buildings against unwanted draughts we seal in the moisture which exists in the air inside. This moisture coming from everything from breathing to washing is absorbed by the air until it reaches 100% relative humidity (RH). It can hold no more water at this temperature. It then condenses on whatever cold surfaces it can find like window glass, any cold – bridging points in the building or in the least ventilated areas. Constant slight condensation, combined with dust will result in mould. The primary objective of any ventilation system is to prevent condensation and mould build-up, which is detrimental to both the building and its occupants.

The HRV system does this by taking air in from outside at typically 3ºC and 90%RH. This air is raised to 18ºC and hence the RH reduced to 42%. This means that the air is able to absorb any water generated as it moves from inlet points in bedrooms to outlet points in bathrooms. By the time the air is expelled through the HRV unit its RH would typically have increased to 65% from 42%. Typical cycle time is two hours per air change.

Infiltration of cold air can represent 50% of the total winter heating load in a building. Total heating costs in an airtight building can be 40% less than in a typical leaky building. A well-designed building uses a controllable ventilation system either by mechanical or natural means.